Ethernet Controller vanishes from Device Manager on reboot

I have a very strange problem on my Windows 7 Ultimate (64) system. My Lan Connection is via a Marvel Yukon 88E8053 PCI-E Gigabit Ethernet Controller on a ASUS P5W DH Deluxe MB. Usually when I shut down for the night, I only put the system in SLEEP mode, thus each day I wake it up and all is well. But occasionally I have to shut down or reboot the computer, like after installing updates...